The present invention relates to communications systems and methods, in particular, to radiotelephone communications systems and methods.
Cellular radiotelephone systems are commonly employed to provide voice and data communications to a plurality of subscribers. For example, analog cellular radiotelephone systems, such as designated AMPS, ETACS, NMT-450, and NMT-900, have been deployed successfully throughout the world. More recently, digital cellular radiotelephone systems such as designated IS-54B in North America and the pan-European GSM system have been introduced. These systems, and others, are described, for example, in the book titled Cellular Radio Systems by Balston, et al., published by Artech House, Norwood, Mass., 1993.
FIG. 1 illustrates a typical terrestrial cellular radiotelephone communication system 20 as in the prior art. The cellular radiotelephone system may include one or more radiotelephones 21, communicating with a plurality of cells 36 served by base stations 23 and a mobile telephone switching office (MTSO) 25. Although only three cells 36 are shown in FIG. 1, a typical cellular network may comprise hundreds of cells, may include more than one MTSO, and may serve thousands of radiotelephones.
The cells 36 generally serve as nodes in the communication system 20, from which links are established between radiotelephones 21 and the MTSO 25, by way of the base stations 23 serving the cells 36. Each cell will have allocated to it one or more dedicated control channels and one or more traffic channels. The control channel is a dedicated channel used for transmitting cell identification and paging information. The traffic channels carry the voice and data information. Through the cellular network 20, a duplex radio communication link 32 may be effected between two mobile stations 21 or between a radiotelephone 21 and a landline telephone user 33. The function of the base station 23 is commonly to handle the radio communication between the cell and the mobile station 21. In this capacity, the base station 23 functions chiefly as a relay station for data and voice signals.
As illustrated in FIG. 2, satellites 110 may be employed to perform similar functions to those performed by base stations in a conventional terrestrial radiotelephone system, for example, in areas where population is sparsely distributed over large areas or where rugged topography tends to make conventional landline telephone or terrestrial cellular telephone infrastructure technically or economically impractical. A satellite radiotelephone system typically includes one or more satellites 110 which serve as relays or transponders between one or more earth stations 130 and radiotelephones 21. The satellite communicates with radiotelephones 21 and earth stations 130 over duplex links 170. The earth station may in turn be connected to a public switched telephone network 140, allowing communications between satellite radiotelephones, and communications between satellite radio telephones and conventional terrestrial cellular radiotelephones or landline telephones. The satellite radiotelephone system may utilize a single antenna beam covering the entire area served by the system, or, as shown, the satellite may be designed such that it produces multiple minimally-overlapping beams 150, each serving distinct geographical coverage areas 160 in the system""s service region. A satellite 110 and coverage area 160 serve functions similar to that of a base station 23 and cell 36, respectively, in a terrestrial cellular system.
Traditional analog radiotelephone systems generally employ a system referred to as frequency division multiple access (FDMA) to create communications channels. As a practical matter well-known to those skilled in the art, radiotelephone communications signals, being modulated waveforms, typically are communicated over predetermined frequency bands in a spectrum of carrier frequencies. These discrete frequency bands serve as channels over which cellular radiotelephones communicate with a cell, through the base station or satellite serving the cell. In the United States, for example, Federal authorities have allocated to cellular communications a block of the UHF frequency spectrum further subdivided into pairs of narrow frequency bands, a system designated EIA-553 or IS-19B. Channel pairing results from the frequency duplex arrangement wherein the transmit and receive frequencies in each pair are offset by 45 Mhz. At present there are 832, 30-Khz wide, radio channels allocated to cellular mobile communications in the United States.
The limitations on the number of available frequency bands presents several challenges as the number of subscribers increases. Increasing the number of subscribers in a cellular radiotelephone system requires more efficient utilization of the limited available frequency spectrum in order to provide more total channels while maintaining communications quality. This challenge is heightened because subscribers may not be uniformly distributed among cells in the system. More channels may be needed for particular cells to handle potentially higher local subscriber densities at any given time. For example, a cell in an urban area might conceivably contain hundreds or thousands of subscribers at any one time, easily exhausting the number of frequency bands available in the cell.
For these reasons, conventional cellular systems employ frequency reuse to increase potential channel capacity in each cell and increase spectral efficiency. Frequency reuse involves allocating frequency bands to each cell, with cells employing the same frequencies geographically separated to allow radiotelephones in different cells to simultaneously use the same frequency without interfering with each other. By so doing, many thousands of subscribers may be served by a system of only several hundred frequency bands.
Another technique which may further increase channel capacity and spectral efficiency is time division multiple access (TDMA). A TDMA system may be implemented by subdividing the frequency bands employed in conventional FDMA systems into sequential time slots, as illustrated in FIG. 3. Although communication on frequency bands f1-fm typically occur on a common TDMA frame 310 that includes a plurality of time slots t1xe2x80x94tn, as shown, communications on each frequency band may occur according to a unique TDMA frame, with time slots unique to that band. Examples of systems employing TDMA are the dual analog/digital IS-54B standard employed in the United States, in which each of the original frequency bands of EIA-553 is subdivided into 3 time slots, and the European GSM standard, which divides each of its frequency bands into 8 time slots. In these TDMA systems, each user communicates with the base station using bursts of digital data transmitted during the user""s assigned time slots. A channel in a TDMA system typically includes one or more time slots on one or more frequency bands.
Because it generally would be inefficient to permanently assign TDMA time slots to a radiotelephone, typical radiotelephone systems assign time slots on an as-needed basis to more efficiently use the limited carrier frequency spectrum available to the system. Therefore, a critical task in radiotelephone communications is providing a radiotelephone with access to the system, i.e., assigning time slots corresponding to a voice or data channel to a radiotelephone when it desires to communicate with another radiotelephone or with a landline telephone or conventional cellular radiotelephone via the PSTN. This task is encountered both when a radiotelephone attempts to place a call and when a radiotelephone attempts to respond to a page from another radiotelephone or conventional telephone.
Access to a radiotelephone communications system may be provided in a number of ways. For example, a polling technique may be utilized whereby a central or base station serially polls users, giving each an opportunity to request access in an orderly fashion, without contention. However, serial polling tends to be impractical for radiotelephone systems because typical radiotelephone systems may have hundreds, if not thousands, of users. Those skilled in the art will appreciate that serially polling this many users can be extremely inefficient, especially when one considers that many of the users may not desire access at all, or may not desire access at the particular moment they are polled.
For this reason, radiotelephone systems typically use random access techniques, whereby a radiotelephone desiring a voice or data channel randomly sends an access request to the base or hub station, which the central or base station may acknowledge by establishing a communications channel to the requesting radiotelephone, if available. An example of a random access technique for a TDMA radiotelephone communications system is that used in the GSM system. In the GSM system, a set of Common Control Channels (CCCHs) is shared by radiotelephones in the system and includes one or more Random Access Channels (RACHs).
If a radiotelephone desires access, the radiotelephone typically transmits a random access channel signal, typically including the radiotelephone""s identification and an identification of the telephone the radiotelephone desires to contact, in what is often referred to as a xe2x80x9cRACH burst.xe2x80x9d As illustrated in FIG. 4A, a RACH burst 410 typically contains several fields, including a plurality of guard bits 420, a sequence of synchronization bits 430, and a sequence of information bits 440. The guard bits 420 are used to prevent overlap of communications occurring on adjacent time slots, as discussed below. The synchronization sequence 430 is used by the receiving station to synchronize with the RACH burst, in order to decode the information contained in the information sequence 440. The information sequence 440 may also include a number of sub-fields, for example, a random reference number field 450 which serves as a xe2x80x9ctagxe2x80x9d for identifying a particular random access request from a particular radiotelephone, as illustrated in FIG. 4B.
In a GSM system, a RACH is a dedicated TDMA time slot on a carrier frequency band, used by radiotelephones to request access to the communications system. Radiotelephones typically time their RACH bursts to fall within an assigned TDMA time slot for the RACH, for example, by waiting a predetermined period after a transition in a synchronization signal transmitted by the base station and then transmitting the RACH burst.
However, because radiotelephones conventionally use a common TDMA time slot for transmitting RACH burst, there is probability of collisions between access requests which are transmitted simultaneously or nearly simultaneously by neighboring radiotelephones. To deal with these collisions, the base station typically implements some form of contention-resolving protocol. For example, the station may refuse to acknowledge simultaneous requests, requiring a requesting radiotelephone to reassert its request if it continues to desire access after failing to establish a channel. Contention-resolving protocols may also use a variety of predetermined delays and similar techniques to reduce the likelihood of radiotelephones engaging in repeated collisions subsequent to a first collision. Contention logic used in the European GSM system is described in The GSM System for Mobile Communications published by M. Mouly and M. B. Pautet, 1992, at pages 368-72. Although these contention-resolving protocols may compensate for access failures, they typically do so by incurring additional transmission and processing overhead.
In addition to colliding with other RACH bursts, a RACH burst may overlap other TDMA time slots, causing undue interference on channels using those slots. Before requesting a channel, a radiotelephone may be only roughly synchronized with the base station TDMA frame, for example, by aligning its internal time reference with the synchronization signal transmitted by the base station in an open loop fashion. Finer synchronization, however, typically occurs only after the base station acknowledges the radiotelephone""s request for access and provides the radiotelephone with signals which allow the propagation delay between the radiotelephone and the base station to be determined. With this information, the radiotelephone can adjust its TDMA bursts to prevent collision with bursts from other radiotelephones arriving at the base station on adjacent TDMA slots.
However, a radiotelephone requesting access prior to such synchronization generally suffers from a time ambiguity with respect to other TDMA bursts in the system, because propagation delay varies with position in the coverage area. FIG. 5 illustrates timing relationships between a first radiotelephone, closely synchronized and communicating with the base station over a TDMA voice channel, and a second radiotelephone located a distance from the base station which desires access to system. Because the second radiotelephone is only roughly synchronized, its internal timing may be significantly skewed with respect to the TDMA frame of the base station, as illustrated. Uncompensated, this time skew may cause, for example, a RACH burst 510 transmitted by the second radiotelephone to have a significant overlap 520 with voice or data communications transmitted by the first radiotelephone on an adjacent time slot. This overlap may cause undesirable interference and diminish communications quality.
As illustrated in FIG. 6, conventional terrestrial TDMA cellular radiotelephone systems may compensate for this problem by incorporating guard time or guard bits 610 in each TDMA slot, typically preceding data bits 620 which carry synchronization, voice, data or other information. Guard bits are inserted in each time slot, during which the receiving unit disregards incoming signals because they may be corrupted by overlapping RACH bursts and other sources of interference. Because the maximum time ambiguity in a terrestrial radiotelephone system tends to be relatively small with respect to a TDMA frame, the number of guard bits needed to ensure acceptable signal quality typically is small. For example, the GSM system incorporates approximately 68.25 guard bits in each time slot to ensure that RACH bursts from a radiotelephones as far as 35 kilometers away from the base station will not cause undue interference on other TDMA slots.
Using guard times or bits to prevent overlap of RACH bursts tends to be impractical for satellite TDMA radiotelephone systems, however, because the large area covered by a typical satellite beam and the large signal propagation distance from the satellite to the radiotelephone can combine to create time ambiguities far larger than those experienced in conventional terrestrial TDMA cellular radiotelephone systems. For example, a radiotelephone communications signal in a satellite beam having a coverage area of an approximate 500 kilometer radius may have a differential propagation delay approaching 6 milliseconds for a radiotelephone located at the periphery of the coverage area, resulting in a comparable time ambiguity for RACH bursts. As a typical TDMA time frame may be only tens of milliseconds long and have a slot length of only a few microseconds, the number of guard bits needed to prevent interference from unsynchronized RACH bursts can be of a magnitude approaching the duration of an entire TDMA frame, and far longer than an individual time slot. Increasing the TDMA frame length and the time slot length to provide a sufficient number of guard bits generally is not a practical alternative, as this approach would tend to reduce the potential information rate of the communications channels.
A technique for providing access to a TDMA satellite radiotelephone communications system has been proposed in U.S. patent application Ser. No. 08/629,358, filed Apr. 8, 1996, assigned to the assignee of the present invention, which includes using a dedicated RACH carrier frequency band to communicate random access channel radiotelephone communications signals. As illustrated in FIG. 7, for a system using a dedicated RACH carrier frequency band, a RACH message can be communicated during any time slot 720 in a TDMA multiframe 710 on the band and avoid overlap with traffic channels as might occur in a slotted RACH system such as GSM. However, access request collisions may still occur when two radiotelephones transmit RACH signals simultaneously on the RACH carrier frequency band.
Because of the power limitations of satellite transponders and mobile units, mobile satellite communications desirably are dominated by line-of-sight signal components. Under poor transmission conditions e.g., when the mobile unit antenna is not properly deployed or is obstructed by objects such as buildings, reflected waves may dominate line of sight waves, resulting in Rayleigh fading and poor communications quality, to the point that the mobile unit may have difficulty monitoring the paging channel to detect incoming calls. For this reason, some mobile satellite systems employ an enhanced-margin xe2x80x9cshort message servicexe2x80x9d (SMS) in which a short, high margin alphanumeric message is supplied to a mobile unit to indicate an incoming call. After receiving the short message, the mobile user may then move to a more favorable location to establish communications. The higher margin for the short message typically is achieved by transmitting at a power level higher than normally used for traffic channels, repeating the short message to allow integration at the receiver in the mobile unit, encoding to increase signal to noise ratio, or a combination thereof. Higher-margin SMS systems are described in U.S. patent application Ser. No. 08/626,182, filed Mar. 29, 1996, assigned to the assignee of the present application.
Short message communication may be unilateral, or the mobile unit may respond with an acknowledgement signal to indicate reception of the short message. Typically, the short message acknowledgment signal is transmitted over the RACH carrier frequency band. To improve the chances of the acknowledgment reaching the central station, it may be transmitted using increased power, bit repetition and coding, i.e., as a high margin RACH (HMRACH). Unfortunately, using increased message repetition generally increases the number of HMRACH bursts, and thus increases the burst rate on the random access channel carrier frequency band. The increased burst rate can in turn increase the probability of collisions on the RACH band. For example, for an unslotted ALOHA random access scheme, the probability of collision Pcol is given by:
Pcol=1xe2x88x92exe2x88x922Tslot*Rburst
where Tslot is the slot duration for a TDMA frame and Rburst is the average rate at which RACH bursts arrive at a receiving station. As can be seen from the above equation, as the rate Rburst increases, the probability of RACH collisions also generally increases.
In the light of the foregoing, it is an object of the present invention to provide systems and methods for communicating acknowledgements to short message service (SMS) messages on a random access channel (RACH) carrier frequency band of a radiotelephone communications system, which reduce the probability of collisions between the acknowledgments and access requests.
This and other objects, features and advantages are provided according to the present invention by radiotelephone communications systems, radiotelephones and methods in which access requests are communicated in a first RACH message time window on a RACH carrier frequency band of a radiotelephone communications system and short message acknowledgements are communicated in a second RACH message time window on the RACH carrier frequency band. If the radiotelephone communications system is a time division multiple access (TDMA) radiotelephone communications system which communicates control messages between a central station and a plurality of radiotelephones over a plurality of carrier frequency bands during a plurality of TDMA control channel multiframes, each of which include a plurality of TDMA time slots, the first RACH message time window preferably includes a first set of TDMA time slots in a group of the TDMA control channel multiframes and the second RACH message time window includes a second set of TDMA time slots in the group of TDMA control channel multiframes, thus forming a RACH multiframe including the first RACH message time window and the second RACH message time window. Preferably, the time windows are separated by a guard time, more preferably, a guard time sufficient to prevent collisions of the access requests and the short message acknowledgements. Reduced probability of collisions between short message acknowledgments can be provided by transmitting the short message acknowledgments encoded according to a predetermined short message codes, preferably spreading codes uniquely assigned to paging groups such that a particular spreading code will be used by a limited number of radiotelephones during a given RACH multiframe.
In particular, according to the present invention, a radiotelephone communications system includes a central station and a plurality of radiotelephones. Short message communicating means communicates alphanumeric messages from the central station to the plurality of radiotelephones. Access request communicating means communicates access requests from the plurality of radiotelephones to the central station over a random access channel (RACH) carrier frequency band during a first RACH message time window, the access requests representing requests for access to the radiotelephone communications system. Short message acknowledgement communicating means, responsive to the short messages, communicates short message acknowledgements from the plurality of radiotelephones to the central station over the RACH carrier frequency band during a second RACH message time window in response to short messages communicated to the radiotelephones.
Preferably, the first and second RACH message time windows are separated by a guard time, preferably a guard time sufficient to prevent collision of the access requests and the short message acknowledgements. The system also preferably includes time division multiple access (TDMA) control message communicating means which communicates control messages between the central station and the plurality radiotelephones over a plurality of carrier frequency bands during a plurality of TDMA control channel multiframes, each of the TDMA control channel multiframes including a plurality of TDMA time slots. The access request communicating means preferably communicates access requests during a first RACH message time window including a first set of TDMA time slots in a group of the TDMA control channel multiframes, and the short message acknowledgment communicating means preferably communicates short message acknowledgements during a second RACH message time window including a second set of TDMA time slots in the group of TDMA control channel multiframes. A RACH multiframe is thereby provide on the RACH carrier frequency band, the RACH multiframe including the first RACH message time window and the second RACH message time window.
According to a preferred embodiment of the present invention, the radiotelephone communications system includes a radiotelephone communications medium over which radiotelephone communications signals are communicated between the central station and the plurality of radiotelephones. Each of the plurality of radiotelephones includes short message acknowledgement transmitting means for transmitting a radiotelephone communications signal representing a short message acknowledgement encoded according to a predetermined short message code, in the radiotelephone communications medium. The central station includes means for receiving a radiotelephone communications signal from the radiotelephone communications medium, and means for decoding the received second radiotelephone communications signal according to the predetermined short message code to thereby recover a short message acknowledgement. The short message communicating means may communicate a short message including the predetermined short message code from the central station to the radiotelephone. To provide for reduced likelihood of collisions of short message acknowledgements, the short message codes may be uniquely assigned to paging groups of radiotelephone""s. The short message communicating means may communicate short messages including a unique short message group assigned to a paging group to a predetermined number of radiotelephones, preferably only one, in a paging group during a RACH multiframe.
A radiotelephone according to the present invention includes short message receiving means for receiving short messages from central station and access request transmitting means for transmitting access requests from the radiotelephone over a random access channel (RACH) carrier frequency band during a first RACH message time window, the access request representing a request for access to the radiotelephone communications system. Short message acknowledgement transmitting means is responsive to the short message receiving means for transmitting short message acknowledgements from the radiotelephone over the (RACH) carrier frequency band during a second RACH message time window, each of the short message acknowledgments being transmitted in response to receipt of a short message from the central station at the radiotelephone. The access request transmitting means preferably transmits access requests during a first RACH message time window including a first set of TDMA time slots in a group of the TDMA control channel multiframes. The short message acknowledgment transmitting means preferably transmits short message acknowledgements during a second RACH message time window including a second set of TDMA time slots in the group of TDMA control channel multiframes of a RACH multiframe on the RACH carrier frequency band, the RACH multiframe including the first RACH message time window and the second RACH message time window, preferably separated by a guard time. According to a preferred embodiment, the short message acknowledgment transmitting means includes means for transmitting a radiotelephone communications signal representing a short message acknowledgment encoded according to a predetermined short message code, and the short message receiving means includes means for receiving a short message including the predetermined short message code.
According to method aspects of the present invention, access requests are communicated from a plurality of radiotelephones to a central station over a random access channel carrier frequency band during a first RACH message time window, the access requests representing requests for access to the radiotelephone system. Short message acknowledgements are communicated from the plurality of radiotelephones to the central station over the random access channel carrier frequency band during a second RACH message time window, in response to short messages communicated from the central station to the plurality of radiotelephones. According to a preferred method aspect, access requests are communicated during a first RACH message time window including a first set of TDMA time slots in a group of TDMA control channel multiframes, and short message acknowledgements are communicated during a second RACH message time window including a second set of TDMA time slots in the group of TDMA control channel multiframes, to thereby form a RACH multiframe on the RACH carrier frequency band, the RACH multiframe including the first RACH message time window and the second RACH message time window. Preferably, the first and second RACH message time windows are separated by a guard time, preferably one sufficient to prevent collision of the access requests and the short message acknowledgements.
According to another preferred method aspect, the radiotelephone communications system includes a radiotelephone communications medium over which radiotelephone communications signals are communicated between the central station and the plurality of radiotelephones, and the step of communicating short message acknowledgements includes the step of transmitting a radiotelephone communications signal from one of the radiotelephones, the radiotelephone communications signal representing a short message acknowledgement encoded according to a predetermined short message code, in response to communication of a short message to the one radiotelephone. The radiotelephone communications signal is received at the central station, and decoded according to the short message code to thereby recover a short message acknowledgement. The short message code may be previously communicated from the central station to the one radiotelephone. A unique short message code may be assigned to a paging group of which the one radiotelephone is a member, and a short message including the unique short message code may be communicated to the one radiotelephone. To reduce the likelihood of collisions between short message acknowledgments, the unique short message code may be communicated to a predetermined number of radiotelephones in the paging group, preferably only one, during a RACH multiframe.